Columns with side strippings

We understand by distillation complex a countercurrent cascade with branching of flows, with recycles or bypasses of flows. Columns with side stripping or side rectifier and columns with completely connected thermal flows (the so-called Petlyuk columns ) are examples of distillation complexes with branching of flows. A column of extractive distillation, together with a column of entrainer regeneration, make an example of a complex with recycle of flows. Columns of this complex work independently of each other therefore, we do not examine it in this chapter, and the questions of its usage in separation of azeotropic mixtures and questions of determination of entrainer optimal flow rate are discussed in the following chapters. 

Three kinds of distillation complexes with thermal coupling flows (with branching of liquid and/or vapor flows) - columns with side stripping, columns with side rectifiers and complexes with full thermal coupling flows, called Petlyuk column -are used in industry at present. 

Columns with side stripping (Fig. 6.12b) are used at refinery beginning with the first decades of the twentieth century (see Watkins, 1979). 

We now turn to the columns with side strippings (for the colunms with side rectifiers, the calculation of minimum reflux mode is carried out the same way). 

Figure 6.15 shows the simple example of separation of a four-component mixture into four pure components in a column with side strippings. As for the columns with side withdrawals of the products, the calculation of minimum reflux mode should be started with determination of the conditions of joining of trajectories of two sections adjacent to feed cross-section. For section ri, the pseudoproduct equals the sum of top and two side products. The minimum reflux mode for the first two-section column is calculated the same way it is done for the corresponding simple column with split 1,2,3 4 (indirect split). In a more general case, when the bottom product contains more than one product component, the intermediate split will be in this column. 

We propose a method of design calculation based on decomposition of the column with side strippings into a system of two-section columns, on rigorous design calculation of each two-section column and on simultaneous design of distillation columns and of system of heat exchange. 

The main task of designing columns with side strippings is the determination of necessary number of plates in each section, of optimal thermal duties on pumparounds and of the rates of steam for stripping or thermal duties on reboilers. 

Figure 7.16. Calculation of crude oil column with side strippings using pinch analysis. LN, light naphtha HN, heavy naphtha LD, light diesel HD, heavy diesel.

The choice between the absence and availability of liquid flow from one two-section column into the other and the determination of optimal flow rate are performed using the same method. Such optimization simultaneously concerns the distillation column and the system of heat exchangers. As a result of such optimization in different two-section columns entering into the column with side strippings, it turns out to be profitable to use the different modification (reboilers or steam stripping, availability or absence of liquid flow between columns) (Fig. 7.16). 

Design calculation of columns with side strippings is carried out on the basis of the algorithm of design calculation of two-section columns and on the basis... 

Optimal cross-sections of joining of previous and posterior two-section columns at calculation by method tray by tray of Petlyuk columns and of columns with side strippings are cross-sections of previous columns closest by composition to points S. ... 

What is the pseudoproduct of the second section above the feed cross-section of the main column with side strippings if its products in the direction downward are... 

The comparison of various distillation complexes and of ordinary flowsheets is given in Tedder and Rudd (1978). Columns with side strippings and side rectifiers, Petl50ik columns, flowsheet with prefracionator, and also some other feasible configurations of two columns were examined. It was shown, in particular, that Petlyuk columns are preferable at big content of average volatile component. 

If a sequence of simple colunms has minimum number of columns (n - 1) and a summary number of reboilers and condensers 2(n - 1), then the column with side strippings or rectifiers (partially coupled sequence) has a summary number of reboilers and condensers n. 

Besides sequences of simple columns, some types of distillation complexes, each of which can replace two adjacent simple columns, were examined in work (Ghnos Malone, 1988). The following complex columns and distillation complexes were examined column with side output above the feed cross-section, column with side rectifier, column with side stripping flowsheet with prefractionator, Petlyuk column top and side flows from the first column into the second one (Fig. 8.3a),... 

Columns with side stripping sections were used for petroleum separation already in the first decades of twentieth century. This choice is quite grounded by the main purposes of designing increase of separability, and decrease of energy and capital expenditures on separation. 

Because petroleum is a mixture with a wide interval of bubble temperatures of components and the required purity of products is not very great, reflux and vapor numbers in the sections are not large. Therefore, the heat brought in is used up mostly not for creation of vapor reflux, but for evaporation of those products that are withdrawn above the feed cross-section. Therefore, the split (direct, indirect, intermediate) has but a weak influence over energy expenditures, but it is very important to exclude multiple evaporation and condensation like in multicolumn sequences of simple columns. Columns with side strippings exclude multiple evaporation and condensation. 

Among multisection distillation complexes, only columns with side strippings bring practically the whole heat into the feeding and bring hve steam into the bottom. Application of pumparounds decreases energy expenditures and recuperates withdrawn heat for heating of petroleum before separation. 

Therefore, column with side strippings, with live steam into the bottom, and with pumparounds is the best distillation complex for petroleum refining. An optimum way of designing such column is discussed in Section 7.5.2. 

Only part of these products is obtained in each unit. Nevertheless, it is not possible to obtain all the products in one column with side strippings because of... 

Therefore, the main succession of petroleum refining includes two main units that of atmospheric distillation and that of vacuum distillation. The lighter products are obtained in the first unit right up to mazut (bottom product of atmospheric column with side strippings) and, in the second unit, the heavier products are obtained right up to tar (bottom product of vacuum column with side strippings). 

The first unit usually consists of one column with side strippings (one-column flowsheet) (Fig. 8.32a) or of superatmospheric column of partial recovery of benzine and of main column with side strippings (two-column flowsheet) (Fig. 8.32b). 

For petroleum mixtures, their peculiarities predetermine the choice of the main distillation complex (column with side strippings) and of main sequence (atmospheric and vacuum columns), but different modifications are possible for different sets of products and compositions of crude are possible. These modifications may significantly improve the economy of the equipment. 

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